The purpose of this blog is to provide a brief resume of
the types of spectacle lenses available to the patient
for better vision (visual needs).
Here we can discuss and share all about ophthalmic lenses, theories, individual findings, inventions, optical standards, tolerance, errors, complex optics and more..

Friday, July 8, 2011

Optics Terms

Accommodation

The ability of the eye to adjust its focusing power (the crystalline lens) to objects that are closer or farther away.

Additive methods of color production

Mixing of the additive primary colors to produce other colors of the spectrum.

Additive primary colors

Red, blue, and green, which mixed in certain combinations produces the other colors of the spectrum and when all mixed together shows white light.

Angle of Incidence

The angle that a light ray hits a surface.

Angle of Reflection

The angle that a light ray is emitted from the surface.

Angle of Refraction

The angle of change in direction that the light wave makes when it undergoes refraction.

Angular magnification

The magnifying power of a magnifying glass, expressed as the ratios of the angular sizes of an object, normally and through the instrument.

Anisotropic material

Materials such as quartz and ice where the characteristics of it are different depending on which way the light rays travel.

Astigmatism

The rays that enter through a lens focus on different points after leaving it.

Astronomical telescope

A telescope where the final image is inverted, which is usually used by astronomers.

Bragg´s Law

Describes the relationship between the wavelength traveling through a grating and the spacing within the crystal that serves for diffraction.

Center of curvature (C)

The point on the optics axis that corresponds to the center of the sphere at which the mirror forms a section.

Chief (radial) ray

A ray that travels through the center of curvature along the mirror?s surface and is reflected back along its original.

Chief ray

A ray that passes through the center of the lens.

Chromatic aberration

White light is dispersed because of different indexes of refraction within a lens.

Circle of Least Confusion

The location of the least distorted image.

Compound microscope

An instrument composed of two convex lenses that combine to increase magnification.

Concave mirror

The inside of the spherical mirror is the reflecting side (it caves in).

Cones

Part of the composition of the retina; it is cone-shaped. It is more used for intense light that the brain will translate into colors.

Converging (biconvex) lens

The lens surface curves inwards.

Converging mirror

A concave mirror.

Convex mirror

The outside of the spherical mirror is the reflecting side (it bulges out).

Critical angle

The angle of incidence which produces a 90 degree angle of refraction; the light ray does not enter the second medium but travels along the surface between the two mediums.

Crystalline lens

A converging or convex lens inside the eye, located behind the iris and composed of microscopic glassy fibers.

Dichroism

A property that some birefrigent materials exhibit, where one polarized beam of light is absorbed more than the other, and the light that leaves the crystal is plane polarized, or polarized in one direction.

Diffraction

The deviation of light after it passes through a small opening.

Diffraction gratings

A series of parallel, very fine, closely placed slits on a panal that often gives rise to an iridescent effect (i.e. CDs).

Diopters

The reciprocal of the lens, expressed in meters and labeled P.

Dispersion

When light enters a material that has different indexes of refraction and is separated (e.g. prism).

Diverging (biconcave) lens

The lens surface curves outwards.

Diverging mirror

A convex mirror.

Electromagnetic wave

Light

Extraordinary ray

When a material exhibits birefringence, there are two beams of light polarized. The extraordinary ray is the beam that is refracted when passing through the crystal.

Eyepiece (ocular)

The second convex lens of a compound microscope that has a longer focal length.

Far Point

The greatest distance at which the normal eye can see objects clearly, and is taken to be infinity.

Farsightedness (hyperopia)

A disease/condition of the eye where it is unable to see things that are close to it. The physics behind it is that the image is being focused not exactly on the retina, but behind it.

Fermat´s principle

Light always takes the path that requires the shortest amount of time.

Fiber optics

A new field of optics where transparent fibers are used to transmit light across distances.

Focal length (f)

The distance between the focal point of a spherical mirror and its vertex.

Focal lens ray

A ray that passes through the focal point of a converging lens.

Focal point (F)

A point where reflected rays intersect when they are parallel to the optic axis.

Focal ray

A ray that travels through the focal point and is mirrored parallel to the optic axis.

Focal Ray

A ray that appears to travel through the focal point and reflects back in a path parallel to the optic axis.

Geometrical optics

The use of geometry to explain phenomena such reflection and refraction.

Huygen´s Principle

Every point on a light wave can be considered as a source of secondary waves and the line or surface tangent to all these secondary waves defines a new position of the wave front.

Index of refraction

The ratio of the speed of light in a vacuum to the speed of light in a medium.

Interference

When two waves interact and effect each other depending on their wavelengths, places of interaction, and other factors.

Irregular (diffuse) reflection

Reflection that occurs when the reflecting surface is rough; reflected light rays are not parallel even if the rays are at the same angle of incidence.

Isotropic material

Materials such as glass where the characteristics are the same no matter which way the light rays travel.

Lateral magnification

The ratio of an object?s image?s height compared to the object?s actual height.

Law of Reflection

The angle of incidence and angle of reflection of a ray of light are equal.

LCD (Liquid Crystal Display)

Crystals that are melted and flow like liquid but still have the orderly composition that light needs in polarization; often used in calculators and digital watches.

Lens

A slice of transparent material that has one or both sides curved.

Lens Maker´s Equation

An equation that gives the focal length of thin lens in air.

Longitudinal waves

Waves where the particles oscillate from side to side.

Magnification factor

A ratio of the distance of an image as compared to the distance of an object.

Magnifying glass

A single convex lens that inflates the size of an object to allow it to be seen more clearly.

Mie Scattering

Dust, water droplets, and particles of the lower region of the atmosphere cause this kind of scattering, which takes place when the particles that spread the emitted substance emission are bigger than the emitted wavelengths themselves. It is not as wavelength dependent as Rayleigh’s scattering. Mie scattering makes clouds look white.

Near point

The closest position to the eye at which it can see the object clearly.

Near Point

The position closest to the eye at which objects can be seen clearly and depends on the extent the lens can be deformed by accommodation.

Nearsightedness (myopia)

A disease/condition of the eye where it is unable to see things that are at a distance. The physics behind it is that the image is being focused not exactly on the retina, but in front of it.

Newton´s rings

A technique that is used to check the smoothness and equilibrium of lenses through an interference where irregularities in the lens cause distortions.

Nonselective Scattering

This kind of scattering occurs in the lower regions of the atmosphere, but unlike Mie scattering, its intensity is not dependant on wavelength. Nonselective scattering is a major cause of haze.

Objective

The first convex lens of a compound microscope that has a shorter focal length.

Optic axis

The line through the center of a spherical mirror.

Optic nerves

Nerves that transmit light signals and thus images to the brain so that we can see.

Optical activity

The ability of a material to polarize light and then change its direction (i.e. the transparent material used to make 3-D glasses).

Optical flat

A flat plate of glass that is polished to the utmost smoothness.

Ordinary ray

When a material exhibits birefringence, there are two beams of light polarized. The ordinary ray is the beam that passes through the crystal without being deflected.

Parallel lens ray

A ray that is parallel to the lens axis.

Parallel ray

A ray that travels parallel to the optic axis and is reflected through the focal point.

Partially polarized light

The light rays are going in all different directions.

Partially polarized light

The light rays are going in a general direction but not altogether oriented.

Physical (wave) optics

The study of light that takes into account the wave nature of light rays.

Pigment

A substance used to produce color.

Plane mirror

A mirror with a flat surface.

Plane wave front

A wave front in three dimensions as opposed to a linear plane (in two dimensions)

Polarization

A specific orientation of transverse light waves.

Polarization by refraction

When light is refracted continuously off a stack of glass plates, instead of only one, and thus polarization increases so that the intensity of the light goes up as well.

Polarized light

The light rays are oriented in the same direction.

Polarizing angle (Brewster angle)

When light hits a surface such as glass, it is usually partially reflected and partially refracted depending on the angle of incidence. This causes the light ray to become either polorized, partially polarized, or unpolarized. If the angle of incidence is exactly 90 degrees, this results in complete polarization of the beam and is known as the Brewster angle.

Radial Ray

A ray that travels through the center of the sphere and is reflected back along the path it came.

Radial Ray

A ray that travels through the center of the sphere and is reflected back along the path it came.

Radius of Curvature (R)

The distance between the vertex and the center of curvature.

Ray

A line drawn perpendicular to a series of wave fronts and pointing in the direction that the wave is going.

Rayleigh criterion

An accepted condition of resolution that states that if two images have coinciding diffraction maximum and minimum patterns, they are said to be resolved.

Rayleigh scattering

The equation that shows the relationship between scattering and the wavelengths of the light rays; the bigger the wavelength, the less the scattering.

Real image

An image that is formed when light rays are reflected by a spherical mirror from the object to the eye; light actually radiates from such images (e.g. movie screen).

Reflecting telescope

A telescope that uses a single large, concave, parabolic mirror. Because of its parabolic nature, the mirror does not have spherical or chromatic aberration and only one surface needs to be prepared and polished.

Reflection

When light is absorbed and then emitted again.

Reflection grating

When light is transmitted through an extraordinarily fine diffraction grating that is also photosensitive and causes a colorful iridescent effect.

Refracting telescope

A refracting telescope works similarly to a compound microscope and also magnifies objects. It uses convex lenses and the phenomenon of refraction. It too has an objective and an eyepiece. To attain the greatest magnification, the focal length of the objective should be maximized and that of the eyepiece should be minimized. Two types of refracting telescopes are astronomical and terrestrial telescopes.

Refraction

When the direction of a light ray changes as it passes through different mediums.

Regular (specular) reflection

Reflection that occurs when the reflecting surface is smooth; reflected light is parallel if the rays are at the same angle of incidence.

Resolution

1) Separating something into its basic components (white light into colors). 2) The specific detail that an image can be tuned to; the clearness of an image.

Resolving power

The minimum distance that a lens needs between two points so that the images can be resolved.

Retina

An area that is photo-sensitive within the eye, whose surface is in contact with light rays. In back of this surface runs many optic nerves that send signals to the brain.

Rods

Part of the composition of the retina; it is rod-shaped, hence the name. It is more sensitive to light and can help one to see when the surrounding is dim (twilight vision).

Scattering

Light may be absorbed and re-radiated from the particles of the medium that it travels through (i.e. air), which is the reason why the sky is blue.

Snell´s Law

A law that shows the relationship between angles of incidence and angles of refraction for a light wave.

Spectrum

The organization of emitted energy in wave-form according to wavelength.

Spherical aberration

The effect that the greater distance between the incident and the axis results in a greater distance between the mirrored ray and focal point.

Spherical mirror

A surface that reflects with spherical geometry.

Spherical mirror equation

An equation that relates the distances that a ray travels and the focal point.

Subtractive method of color production

Color is perceived by a method known as subtraction, where the color seen is the color not reflected by the pigment.

Subtractive primary pigments

Cyan, magenta, and yellow, which are the complementary colors of the additive primary colors and combine together to form black.

Terrestrial telescope

A telescope where the final image is right-side up, which is more useful for those viewing objects on Earth.

Total internal reflection

The condition in refraction where light does not pass through the surface but is internally reflected; the angle that the light is refracted is greater than the critical angle.

Transmission axis (polarization direction)

When certain birefrigent materials are used, such as polymers, the molecules can orient themselves into chains. The transmission axis is perpendicular to this chain.

Transmission grating

When light is transmitted through a diffraction grating, often caused by an extremely fine wire grating.

Transverse waves

Waves where the particles ondulate up and down.

Unpolarized light

The light rays are going in all different directions.

Vertex

Where the optical axis intersects the spherical mirror?s surface.

Virtual image

An image that is formed when the light rays are reflected by a planar mirror from the object to the eye, so that the object actually appears to be behind the mirror; light does not actual radiate from such images.

Wave front

A line or surface defined by adjacent portions of a wave that are in phase.

Young´s double-slit experiment

Young, an English scientist, proved by sending light through a slitted panel onto a screen that light rays have wave properties. It was also through this experiment that Young measured wavelengths.